Method of making sheet glass

ABSTRACT

Sheet glass of commercial quality is made by a Pittsburgh Process without using a shutoff. Instead, means are used to seal the kiln from the tank, and a water-cooled pipe is submerged in the glass and lowers suitably the flow; throughout ratio. the submerged pipe has a diameter of 1.2 to 5 inches and it is located so that it obstructs the return flow, being positioned so that its top is not above the mid-point of the depth of the glass, and so that its bottom it at least one-fifth of the depth of the glass above the bottom. In this way of benefits of having no shutoff are obtained: lower costs, and an avoidance of defects in the products caused by shutoff.

sept. 25, 1973 G. E. KUNKLE METHOD OF MAKING SHEET GLASS Filed Feb. 2, 1971 i INVENTOR GfLD E- KUN/(Lf ATTORNEY! United States Patent 3,761,236 METHOD F MAKING SHEET GLASS Gerald E. Kunkle, New Kensington, Pa., assignor to PPG Industries, Inc., Pittsburgh, Pa.

Continuation-impart of abandoned application Ser. No.

826,860, May 22, 1969. This application Feb. 2, 1971,

Ser. No. 111,907

Int. Cl. C03b 15/14, 15/02, 5/16 U.S. Cl. 65-83 1 Claim ABSTRACT 0F THE DISCLOSURE Sheet glass of commercial quality is made by a Pittsburgh Process without using a shutoff. Instead, means are used to seal the kiln from the tank, and a water-cooled pipe is submerged in the glass and lowers suitably the ow; throughout ratio. The submerged pipe has a diameter of 1.2 to 5 inches and it is located so that it obstructs the return flow, being positioned so that its top is not above the mid-point of the depth of the glass, and so that its bottom is at least one-fifth of the depth of the glass above the bottom. In this Way the benefits of having no shutoff are obtained: lower costs, and an avoidance of defects in the products caused by shutoff.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-impart of my earlier filed copending aplication Ser. No. 826,860, filed May 22, 1969, now abandoned.

BACKGROUND 0F THE INVENTION (l) Field of the Invention This invention relates to a method of making sheet glass by a Pitsburgh Process, and in particular, to a method whereby the cost of producing sheet glass is notably lessened without any corresponding decrease in the quality of the product.

(2) Description of the prior art 'Since about the 1920s, there has been practiced the Pittsburgh Process, wherein sheet glass is made by drawing vertically from a kiln about 30 inches deep or more and containing a totally submerged drawbar of refractory material a sheet glass of about 8O to 110 inches wide. In the Pittsburgh Process as it has been practiced hitherto, there has been customarily used, upstream of the location of the drawbar and of the meniscus, a partially submerged shutoff of refractory material. In use, such shutoff members exhibit Wear and must be replaced from time to time. The shutoff tends to affect the ow pattern of the glass, disrupting it from strictly laminar flow and having as a consequence a deleterious effect upon the quality of the sheet-glass product. As happens with any refractory member submerged in the flow path of the glass, some of the member erodes and breaks off during use and thus contaminates the product. In the Pitsburgh Process, the shutoff serves several useful purposes, including diminishing the ow:throughput ratio and preventing sulphate-bearing gases from the melting tank from reaching the drawing kiln and the vicinity of the meniscus. It is considered likely that if such sulphate-bearing gases enter the drawing kiln,

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they will have an adverse effect upon the service life of the coolers therein.

It nearly invariably happens, moreover, that the abovementioned movement of gases from the tank into the kiln take place, in the Pittsburgh Process, unless a shutoff is used. The tank has a headspace that is, in effect, substantially closed, and gaseous fuels are being fed into such headspace to burn and heat the batch, so that the tank headspace upstream of the shutoff is at an absolute pressure a litle above the atmospheric. On the other hand, the kiln is located directly below the drawing machine, which extends vertically for about two or tree building-stories and thus operates as a chimney or smokestack-as the less dense, hot gases in the headspace of the kiln rise, they create a kinetic effect that causes even more of them to be withdrawn from the kiln than would occur merely from consideration of the tendency of hot gases to rise because of differences in density. This means that the headspace of the kiln in the Pittsburgh Process operates at a slight partial vacuum, say, on the order of one or two inches of water. In order to practice the Pitsburgh Process and obtain its advantages, the state of affairs indicated above is necessary.

It is true that the Colburn Process for making sheet glass has been operated for many years, and it does not use a shutoff. On the other hand, it does not use a vertical drawing machine that creates the chimney or smokestack effect; instead, the glass is drawn upward only a short distance and is then bent over a bending roll to a substantially horizontal plane. The bending operation must take place while the glass is relatively plastic or deformable, and we consider that this necessarily places a limitation on the quality of the glass that can be produced with such a process, because the bending roll, coming into contact with the glass While it is so deformable, necessarily introduces some distortions. The measures that need to be taken so that contact between glass and the bending roll in the Colburn Process is avoided (e.g., creating a gas film about the bending roll) are disadvantageously costly and are possibly of dubious effectiveness. All things considered, previous noshutof Colburn operations are not believed to suggest to a person of ordinary skill in the art the feasibility of operating a Pittsburgh Process without a shutoff.

To the extent that a Pittsburgh Process is operated with a h'igh iiow:throughput ratio, the process is relatively inefiicient. In other Words, it is generally undesirable to be consuming fuel in the melting tank at such a rate as to generate a substantial forward flow and a substantial return flow, with the throughput (amount of glass removed via the meniscus) being relatively small in comparison with the volumes of the above-mentioned forward and return flows. As indicated above, one of the purposes of a shutoff is to reduce the flou/:throughput ratio to a manageable and desirable level, such as about 2:1. If no shutoff is used, and no other measures are taken, this ratio is likely to be 3:1 or higher.

SUMMARY OF THE INVENTION In accordance with the invention, a Pittsburgh Process is practiced in which air-curtain means or similar noncontacting means are provided to seal the melting tank away from the drawing kiln, and at the same time, there is also provided, submerged within the molten glass and in such a position as to act to impede substantially and in an appropriate manner the forward flow and/or the return ow. When the instant invention is practiced, continual replacement of the refractory shutoff member hitherto customarily used is avoided, and at the same time, laminar flows of glass are more surely obtained, so that as a result, the quality of the product is improved.

DESCRIPTION OF THE DRAWING A complete understanding of the invention may be obtained from the foregoing and following description thereof, taken together with the appended drawing, in which:

The sole figure is a schematic representation, in sectional elevation view, of one embodiment of equipment according to, and used in the practice of, the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figure, there is shown a drawing kiln at 2 and a region 4 through which molten glass 6 passes to the kiln 2. schematically indicated at 8 are air-curtain means that serve to prevent gases from the headspace region from passing into the drawing kiln, as at 12, and thereby causing premature failure of the main coolers 14. The means 8 may take any suitable form, such as the air-curtain seal of Belgian Pat. No. 721,743, it being important for the purposes of the invention that the gas-seal means used be such that all of its solid elements are spaced from the surface of the molten glass, i.e., not in contact therewith or partly submerged therein. The gases supplied through the means 8 are preferably of such temperature as not to cool the glass 6 unduly. Moreover, the kinetic energy with which the gases from the means 8 impinge upon the glass 6 is preferably such that no substantial disruption of the laminar flow of the glass 6 occurs.

In the inlet end 9 of the tank 11, there are provided gaseous-fuel feed means, indicated at 15.

In accordance with the invention, there is provided, in addition to the gas-seal means 8, a pipe 16 that extends transversely of the main path of flow of glass 6 and is water-cooled. The pipe 16 may be made of AISI Type 446 stainless steel, and it is preferably supplied by suitable means (not shown) with cooling water, so as to effect the extraction of suitable amount of heat. The pipe 16 may have a diameter of about 2 inches, and be capable of extracting about 225,000 B.t.u. per hour. In the embodiment of the invention shown in the figure, the pipe 16 is located so that it impedes the forward flow of the glass 6. It is, for example, totally submerged in the molten glass, with its center lying about 9 inches below the surface of the glass if the glass is about 30 inches deep. As indicated by the arrow 20, and as is usual in the Pittsburgh Process, the glass 6 has an upper region characterized by forward flow; the arrow 22 indicates the rearward or return iiow of the glass 6. For such a process wherein there is drawn from the kiln of about 50 tons per day of glass, it is usual for the forward flow to be on the order of 116 tons per day, assuming that a shutoff is used. If the shutoff is removed, the forward flow and the return tiow are on the order of 140 tons per day. Although the removal of the shutoff has certain advantages as respects permitting glass to reach the meniscus while it is hotter and while its flow is more direct and more nearly laminar, omitting a shutoff is not ordinarily practiced because of consequent inefficiency in the melting operation, and because of danger of premature failure of the main coolers 14 or the contamination of the glass because of the buildup on the main coolers 14 of products of reaction with sulphates contained in the melting-tank gases, possibly followed by the contamination of the surface of the sheet-glass product with such reaction product.

As indicated at 24, the submerged pipe 18 has a thereon buildup of hardened glass about 1 to 2 inches thick; this, taken together with the diameter of the pipe, serves as a substantial barrier to the forward flow indicated by the arrow 22, and it brings the fiow2throughput ratio accordingly into the desired range generally on the order of about 2:1 or a little bit more. At the same time, the gas-curtain means 8 serves to prevent sulphate-laden gases from the melting tank from causing trouble with the main coolers 14. Taken together, and, the features depicted in the figure indicate a method of making vertically drawn sheet glass, in accordance with which there are obtained benefits of operating without a shutoff (a more nearly laminar flow of glass, hotter glass reaching the meniscus so as to minimize variations from one side to the other in thickness of the sheet-glass product, avoidance of labor and material costs in connection with replacing or renewing of the shutoff, etc.) and at the same time the flowzthroughput ratio is desirably reduced.

It will be apparent that the pipe 16 may, in another embodiment of the invention, be so located as to impede the return flow, rather than the forward flow. It is apparent that if this is done, care must be taken not to permit the buildup of glass 24 on the pipe to come into contact with the bottom 26. If that happens the bottom 26 begins to be cooled by conduction and a freeze-up condition may result. Fluid-mechanics considerations also indicate that the submerged pipe 16 should be located somewhat above or somewhat below the middle of the depth of the glass in the tank and kiln, since there is near the middle a surface or region of little or no flow and to the extent that the pipe 16 and its glass buildup 24 impinge upon such surface or region, they are relatively ineffective for the purpose of diminishing the flowzthroughput ratio and may in some instances tend to generate undesirable flow disruptions that adversely affect the quality ofthe sheet-glass product.

With the invention, the costs connected with fabricating, installing and replacing the shutoff members hitherto used are avoided. There is no continual erosion of shutoffrefractory material that tends to contaminate the product. The upsets of the drawing process that occur when the shutoff requires replacement are also avoided.

What is claimed is:

1. In a process for converting glass-batch materials to sheet glass, said process comprising the steps of establishing in a tank and a drawing kiln communicating therewith a mass of molten glass produced by melting of said glass-batch materials, a headspace existing above the molten glass in said tank and said kiln, and said kiln containing a totally submerged drawbar and having positioned thereabove a Pittsburgh-type vertical drawing machine,

feeding onto the surface of said molten glass in said tank additional glass-batch material and feeding to the headspace of said tank gaseous fuel for the melting and heating of said glass-batch materials, there being established in said molten glass in an upper portion thereof a forward fiow from said tank towards and into said kiln and in a lower portion thereof a return liow from said kiln towards and into said tank, and there also being established a greater headspace absolute pressure in said tank than in said kiln, the improvement which comprises sealing said headspace of said tank from said headspace of said kiln by directing a stream of hot gas against the surface of said molten glass transversely across the width of said kiln substantially along a line separating said kiln from said tank, said hot gas being maintained at sufficiently high temperature to maintain the surface temperature of said molten glass contacted thereby substantially undiminshed, and

cooling said molten glass at a location substantially between said tank and said kiln lsubstantially beneath 5 where said hot gas is directed against said molten 3,475,152 glass by removing heat from said molten glass be- 1,818,231 neath its surface. 2,077,377 2,140,281 References Cited 3,607,183

UNITED STATES PATENTS Kelly 65-339 X Mambourg 65-202 Drake 65-204 Drake 65--204 Flori 65-83 FRANK W. MIGA, Primary Examiner Diederichs 65--90 X Zellers 65--202 U.S. Cl. X.R. Drake 65-203 Prislan 5 137 X 10 65-137, 203, 204, DIG- 4 Ward 65-204 X 

